The present invention relates to a defect assessing apparatus and method and a semiconductor manufacturing method to make possible detection and assessment of defects in silicon and other wafers including particles on the surface and crystal defects having an influence on semiconductor devices.
Prior Art
Defect measuring methods according to the prior art include irradiation with infrared rays, which can pass through silicon, and detection of the resultant scattered light Such a measuring method requires measurement of the size and depth of each defect, because the impact of a defect varies with its size and depth.
One of the defect measuring techniques available according to the prior art is to cleave a silicon substrate, irradiate it with infrared rays passing through Si crystals in the direction of its cross section (the direction normal to the direction of the surface slope of the sample), and photograph optical images of defects as light points in the Si crystals. This method, called infrared scattering tomography, is described in detail in the Journal of Crystal Growth, Vol. 88 (1988), p. 332. This measuring technique, though it reveals the distribution of defects present in a minute area, requires cleaving of the sample and, being a destructive test, takes time to prepare the sample. As the sample is irradiated with a beam and scanned in the direction normal to the direction of detection, depth resolution can be achieved according to the diameter of the irradiating beam. The resolution is at most approximately the wavelength of the irradiating light (about 1 .mu.m).
The Japanese Patent Laid-open No. Hei 5-264468 discloses a method by which the depth of each defect is determined by irradiating the sample with infrared rays obliquely, two-dimensionally observing a scattered image from within the sample with an infrared camera, and matching the depth of each part of the scattered image with the position in its field of vision. The depth resolution in this case is determined by optical imaging performance (focal depth), and approximately no finer than the product of the wavelength multiplied by the refractive index, i.e. at most 4 .mu.m.
According to the prior art, disclosed in the Japanese Patent No. Hei 7-294422, laser beams differing in wavelength are brought to incidence on the same surface position of a silicon wafer and, as the penetration depth differs with the wavelength, the difference in the number of defects between the different wavelengths reveals the depth distribution of defects. Thus where, for instance, two different wavelengths are used, if a given defect can be detected by the wavelength of the greater penetration depth, the defect will be within this penetration depth, and if it cannot be detected by the other wavelength of the smaller penetration depth, the defect will be deeper than that penetration depth.
By another conventional method, disclosed in the Japanese Patent No. Hei 6-50902, for measuring defects on a semiconductor wafer surface, the wafer surface is irradiated with a laser beam. In this process, the wafer is rotated, and the scattered light from the wafer surface is condensed by a lens and detected by a detector. The apparatus to implement this process is provided with a frequency band-wise divider for dividing the obtained detection signals into a high frequency band, an intermediate frequency band and a low frequency band; a plurality of analog-to-digital (A/D) converters for digitizing the divided defect detection signals; and a plurality of memories for storing each defect at an address corresponding to its detected position as defect data. Each unit of defect data is processed by a data processing section, mapped on a printer band by band, and the type of defect is differentiated and assessed according to each mapped unit of defect data. This method is intended to differentiate defects by shape and size on the basis of the frequency band of scattered light detection signals generated in a pulse form over time. Measuring methods intended for the inspection of foreign matter on the surface, including this one, generally assesses the size of each piece of foreign matter on the surface according to the intensity of scattered light of one wavelength. When this principle is applied to the assessment of internal defect size, there arises the problem of impossibility to assessing the size of some defects because the scattered light intensity for defects even of the same size attenuates differently depending on their depth.
Devices in a large scale integrated (LSI) circuit are often formed in an area of not more than about 0.5 .mu.m in depth from the silicon surface. Although defects in this area raise the ratio of device failure, defects in a deeper area are often unrelated to device failure. Therefore, the depth resolution of defect measurement should be at least 0.5 .mu.m. Moreover, size assessment should be possible because the impact of a defect on a device differs with the size of the defect. A conventional wafer particle counters, which irradiates a wafer with light of a wavelength absorbable by the wafer and detects scattered light from a defect, can determine neither the size nor the depth because the intensity of scattered light varies with the depth and size of the defect. The method by which depth distribution is assessed from the difference in the number of defects according to the difference between wavelengths in penetration depth involves the problem that a correct result cannot be necessarily obtained because the detectable depth varies with the defect size. Furthermore, since defect size distribution varies with the depth, no correct measurement of defects is possible unless defect sizes are known as differentiated by depth.